Here are four developmental PM tubes.
Each carries a label
which reads "This developmental product is being supplied for experimental purposes. Before
crystallizing any equipment designs, please contact your RCA field representative concerning
future availability. The warranty period for developmental products is 180 days from the date
of shipment by RCA.

This valve has graphite paint covering all the glass except for the input
window. The paint is connected electrically to a base pin via a wire. The type number is C31036, serial K09220,
with D19 written in pen. The number 11336 appears as well but the label is damaged.

This sample has a different internal construction, see below.

Several dynodes are missing from this valve, see the left hand picture. The picture
on the right shows the inside of the other valves in the set.

Apart from the differing internal structure, this valve has black plastic covering the input window. The
label is badly cut making it hard to read the type number, which may be C31036. The serial number
is A00588, and L-16 is written in pen.

This sample carries the type number C7253H10 and the serial A00238. What appears to be
Y9 is written in pen and the number 12362 also appears.

This sample has type number C7253H, serial X08563. There are no other marks.